The present invention relates to color electronic (e.g., LCD) projectors and, in particular, to such a projector that includes equal-length color component paths and a separate projection lens assembly for each.
Color electronic (e.g., liquid crystal display) projectors generate display images and project them onto display screens, typically for viewing by multiple persons or viewers. The display images may be formed by transmitting light from a high-intensity source of polychromatic or white light through or reflected from an image-forming medium such as a liquid crystal display (LCD).
FIG. 1 is a schematic diagram of a prior art multi-path reflective color liquid crystal display projection system 10 that utilizes color separating mirrors 12R, 12BG, and 12G in combination with polarization selective polarizing beam splitters 14R, 14G, and 14B and reflective liquid crystal displays 16R, 16G, and 16B.
Projection system 10 includes a light source 18 that directs white light through a polarizer (or polarization converter) 20 that provides polarized light to a pair of crossed dichroic mirrors 12R and 12BG. Dichroic mirror 12R reflects red light components along a red optical path 22R that is folded by an achromatic fold mirror 24R. Green and blue light passes through mirror 12R. Mirror 12BG reflects blue and green light components along a blue-green optical path 22BG that is folded by an achromatic fold mirror 24BG. Red light passes through mirror 12BG. Mirror 12G reflects green light components along a green optical path 22G and allows the blue light components to propagate along a blue optical path 22B. As a result, mirrors 12R, 12BG, and 12G cooperate to separate polarized red, green and blue light components and deliver them to polarizing beam splitters 14R, 14G, and 14B. The color component images are combined by an X-cube 26 and directed to a projection lens assembly 28.
Each polarizing beam splitter 14 includes a pair of right-angle prisms having their inclined faces positioned against each other with a polarization selective dielectric film (not shown) positioned therebetween. As is conventional for polarizing beam splitters, P-polarized light passes through the dielectric film and S-polarized light is reflected. S- and P-polarizations are conventional nomenclature referring to a pair of orthogonal linear polarization states in which, with regard to a polarization selective dielectric film, S-polarized light can be said to xe2x80x9cglancexe2x80x9d off the film and P-polarized light can be said to xe2x80x9cpiercexe2x80x9d the film. Polarizer 20 transmits the red, green and blue light components as predominantly S-polarized light, so nearly all the light received by polarizing beam splitters 14R, 14G, and 14B is reflected by the dielectric films to reflective liquid crystal displays 16R, 16G, and 16B.
In one implementation, reflective liquid crystal displays 16 are quarter wave-tuned (i.e., with 45xc2x0-60xc2x0 twists) twisted nematic cells and reflect light from each pixel with a polarization that varies according to the control voltage applied to the pixel. For example, when no control voltage is applied (i.e., the pixel is in its relaxed state), the pixel imparts maximum (i.e., a quarter wave) phase retardation that results in a polarization rotation for suitably aligned polarized light. Each pixel imparts decreasing polarization rotation with increasing control voltage magnitudes until the pixel imparts no rotation (i.e., the pixel is isotropic).
In the relaxed state of a pixel, the polarization state is reversed when the light is reflected, so that the S-polarized light becomes P-polarized light. The P-polarized light then passes through the dielectric film of the polarizing beam splitter toward a crossed-combining prism 26 (also known as an X-cube) to be incorporated into the display image. With non-zero control voltages, the pixel reflects the light with corresponding proportions of P- and S-polarizations. Control voltages of greater magnitudes in this example cause greater portions of the light to be reflected with S-polarization, with all the reflected light having S-polarization at the greatest control voltage. The portion of the light with S-polarization is reflected by the dielectric films in polarizing beam splitters 14 back toward the illumination source and are not incorporated into the display image.
Such a multi-path reflective color liquid crystal display projection system 10 suffers from disadvantages that impair its imaging characteristics. One of crossed mirrors 12R and 12BG is actually formed with two mirror halves that are positioned behind and in front of the other of mirrors 12R and 12BG. Proper alignment of the mirror halves is very difficult and rarely achieved. As a consequence, the images reflected by the mirror halves are misaligned, which can result in readily discernible misalignments in the image halves. The relatively common misalignment between the mirror halves introduces, therefore, generally unacceptable image errors that may appear as de-coupled image halves that are improperly joined along an apparent seam.
Similarly, X-cube combiner 26 suffers from manufacturing limitations, such as an inability to perfectly form and join its components. In particular, such imperfections can arise at a central intersection region 29 where the X-cube components meet. Such imperfections are significant because they affect the central, most discernible region of an image.
FIG. 2 shows another prior light valve image projection system 30 as described in U.S. Pat. No. 5,327,270 of Miyatake. Projection system 30 includes three reflective liquid crystal panels 32A, 32B, and 32C that have corresponding polarizing beam splitters 34A, 34B, 34C, quarter wave plates 36A, 36B, 36C, and projection lenses 40A, 40B, 40C, respectively. Dichroic mirrors 42A, 42B, 42C color separate the light from a light source 44.
Color separation by successive dichroic mirrors 42A, 42B, 42C eliminates image errors and artifacts that can be introduced by crossed mirrors 12R, 12BG in projection system 10. Also, separate projection lenses 40A, 40B, 40C eliminate the image errors and artifacts that can be introduced by X-cube 26. To achieve such results, however, projection system 30 employs an in-line arrangement that is bulky and creates optical paths of different lengths for the different color components. The in-line arrangement of projection lenses 40A, 40B, 40C creates relatively large separations between them, thereby imposing relatively large convergence angles that can introduce color component misalignments at the display screen.
Moreover, different path lengths are disadvantageous because the differences causes different magnifications of the xe2x80x98illumination patternxe2x80x99 onto each of the three color channels. When different color channels receive illumination patterns of different magnifications, (e.g., if R illumination is bigger than G and B illumination) the intensity uniformity profiles will be different, and it will be difficult to achieve a uniform white field by superposition.
In accordance with the present invention, an electronic (e.g., LCD) projector combines multiple projection lens assemblies with equal color component optical path lengths to provide improved display images and a compact arrangement. In one implementation, the projector includes a successive pair of angled dichroic mirrors that fold the red and blue color components of light in opposed directions. The green color component of light passes through the dichroic mirrors toward a pixelated electronic light modulator, such as a liquid crystal display, and an associated projection lens assembly. The red and blue color components of light are each folded again to propagate parallel with the green color component toward a pixelated electronic light modulator, such as a liquid crystal display, and an associated projection lens assembly. The separate projection lens assemblies are arranged in a non-linear, close-packed arrangement to receive the color components of light.
The equal lengths of the color component optical paths allow uniform magnifications of the xe2x80x98illumination patternxe2x80x99 onto each of the three color channels. As a result, the color channels receive illumination patterns with generally the same intensity uniformity profiles and in superposition provide a white field with improved uniformity.
In addition, the non-linear, close-packed arrangement of the projection lens assemblies create minimal separations between them, thereby minimizing convergence angles and color component misalignments at the display screen. For example, projection lenses have imperfections in light transmission, especially vignetting or so-called relative illumination deviations, for portions of imaging fields that are away from the lens centers. The minimal separations between projection lens assemblies provided by this invention minimize image defects that can result from such imperfections and provide optimal uniformity and focus and minimal geometric distortion at the final image plane (i.e., the display screen).
Additional objects and advantages of the present invention will be apparent from the detailed description of the preferred embodiment thereof, which proceeds with reference to the accompanying drawings.